Billionaire Democratic philanthropist Tom Steyer, who’s bankrolled two successful California environmental ballot measures, is making an aggressive new political play — putting his money and muscle into a combative Massachusetts Senate race sure to highlight the debate over the Keystone XL pipeline.

The Bay State race pits House Reps. Edward Markey and Stephen Lynch against each other in a whirlwind contest to replace Senator John Kerry, now Secretary of State. Both have only until the April 30 primary to make their case to voters, and are expected to battle it out in an avalanche of paid media. The general election is June 25.

First, take a look at Steyer speaking recently to environmental groups about the Keystone pipeline:

Lynch — a former ironworker for more than a decade — is painting himself as the hero of the working man, with a new ad highlighting his childhood in a South Boston housing project. He was one of 47 Democrats in the House to support a 2011 GOP-backed bill supportive of the pipeline, and directing Obama to expedite the Keystone project.

Markey, who’s been in the House for three decades, has taken a strong position against Keystone.

Here’s where Steyer comes in: in early March, Massachusetts environmentalists made the admittedly unusual move to approach him to create a SuperPAC that would dramatize what they insist are Keystone’s many dangers.

“Given the incredibly harmful role dirty energy money has played in our political system, we are hesitant to suggest that more money in the system is the solution,” wrote Craig Altemose, executive director of the Better Future Project, and his supporters in a letter to Steyer. “But we believe that we are in such a crisis moment that anything we can do to confront this issue head on can, and must, be done.”

Steyer hasn’t hesitated to put his money into environmental issues before; he put $35 million into the successful 2012 ballot measure Prop. 39, which closed a corporate loophole to bring in $500 million in revenue for green energy projects in the state.

Keystone opponents, including Steyer, believe the pipeline is an environmentally damaging project — one that has less to do with providing gas and oil to Americans, and more about sending it abroad to Asia.
Keystone supporters, including Lynch, have said it will create thousands of much-needed permanent jobs, and a road to energy independence.

But on Monday morning, Steyer will publicly challenge Lynch on that front — issuing a formal a letter accusing him of making false claims about Keystone, including how many jobs it will produce. The letter charges that a recent U.S. State Dept. report has “identified only 35 permanent jobs that will be created by Keystone.”

Steyer and environmentalists want Lynch to respond to that claim, and to provide sworn evidence that the Keystone produced oil will stay in the U.S.

They’re giving Lynch until Friday at high noon to respond, according to their letter.

And if he doesn’t, the real campaign begins, the letter says. Steyer backers say he promises ”a robust field effort” to target climate change voters, an education campaign using “guerilla marketing tools” to inform Bay State voters on the issues, and releasing a series of studies and investigative reports on Lynch — and the issue.

All this could get controversial — especially since there’s a pledge in the Massachusetts race to limit SuperPAC spending in traditional ads. But Steyer and his environmental team argue the pledge doesn’t cover field work, and “non-conventional” efforts.